Thermoelectric materials exhibit a predictable relationship between their electrical and thermal performance. Depending on the desired outcome, a thermoelectric apparatus is frequently arranged as either a cooling/heat pump device or as an electrical power generator. To use the material as a cooling device or heat pump, an electric field is applied across the material, typically via an electric circuit. This field generates a thermal gradient according to the Peltier effect so long as it is maintained. Heat can be removed from the hot side of the material by a heat sink, heat exchanger or other heat removing means. When both are done simultaneously, the apparatus then operates effectively as a cooling device or heat pump. Conversely, creating a thermal gradient across the material, such as by application of thermal energy, induces an electric field according to the Seebeck effect. Connecting the material to a circuit aligned with this field causes the apparatus to operate effectively as an electrical power generator.
There are numerous benefits of applying thermoelectric materials to cooling and electrical power generation. The total size of the thermoelectric materials and ancillary parts of a thermoelectric cooling apparatus can be relatively small compared to the size of an equivalent cooling system with typical components. Most typical cooling systems in current production utilize a vapor compression apparatus, which requires at least a compressor, a working fluid, an expansion valve, an evaporator, and a condenser. Likewise, typical power generation facilities utilize large steam turbines to convert heat energy into electricity. A thermoelectric apparatus is smaller because there is less need to house this large, expensive equipment to convert energy from one form to another. This reduces the amount of space necessary to operate a cooling or power generation system, saving valuable functional space for a manufacturer, designer, or builder. Therefore, the compact size of thermoelectric materials makes them useful for many cooling and power generation applications where space is at a premium.
End users also benefit from the virtual maintenance-free operation of a thermoelectric apparatus. Typical systems as described above use mechanical intermediaries to convert energy between a thermal form and an electrical form, while thermoelectric materials convert energy at the material level. Moving parts such as turbines, motors, and pumps, even if reliable, typically require periodic inspection and maintenance to minimize unplanned outages and major repairs. A thermoelectric apparatus has fewer such ancillary parts, and therefore fewer opportunities for system downtime.
Despite the size and maintenance advantages over current systems, thermoelectric materials have to date been limited to certain niche and specialty applications. Key factors limiting widespread adoption of the technology are high production costs combined with the practical size limits of existing thermoelectric materials. The achievable thermoelectric performance of current materials, especially those designed to operate near room temperature, is also inadequate to make thermoelectric cooling systems competitive for many large scale operations, further restricting broad adoption of current thermoelectric technology.
Current thermoelectric materials are best suited for applications with virtually no cost restrictions, a significant size constraint, or where reliability is more critical than efficiency. For example, current applications include those in remote unattended land-based or space-based operations and those applications operating on a very small scale, particularly in the medical and scientific fields.
Thermoelectric cooling is penetrating the consumer market in certain specialty products such as portable refrigeration units and heated/cooled automotive seats, but has yet to reach wide acceptance. Each of these applications have particular reliability or size requirements that make thermoelectric cooling more attractive than standard vapor compression systems. In products where thermoelectric materials have been applied, specialized needs have dictated use of the current generation of materials, despite their limitations.
The performance of a thermoelectric material is characterized by its dimensionless figure-of-merit, ZT:
                              ZT          ≡                                                    σ                ⁢                                                                  ⁢                                  S                  2                                            k                        ⁢                          T              M                                      ,                            (        1        )            
where, σ is the electrical conductivity, S is the Seebeck coefficient or thermoelectric power, TM is the average of the hot and cold side temperatures, and k is the thermal conductivity. As ZT increases, so does the conversion efficiency. While it is desirable to increase ZT by increasing σ or reducing k, there has been limited progress in the ability to beneficially change only one factor without causing a counteracting change in the other.
In Equation 1, thermal conductivity, k has two components. Thermal conductivity is the sum of the electrical contribution, kel and phonon contribution kph, or k=kel+kph. Most advances to date in thermoelectric performance have come from fabricating materials with greatly reduced kph. However, kel and σ are related by the Wiedemann-Franz law, so that an increase in σ typically causes an offsetting increase in kel, resulting in no material change to ZT. A reduction in kel can similarly cause an offsetting decrease in σ. These offsetting changes usually result in no material increase in ZT.